Download Fasting vs dietary restriction in cellular protection

Oncogene (2011) 30, 3305–3316
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REVIEW
Fasting vs dietary restriction in cellular protection and cancer treatment:
from model organisms to patients
C Lee and VD Longo
Andrus Gerontology Center, Department of Biological Sciences and Norris Cancer Center, University of Southern California,
Los Angeles, CA, USA
The dietary recommendation for cancer patients receiving
chemotherapy, as described by the American Cancer
Society, is to increase calorie and protein intake. Yet, in
simple organisms, mice, and humans, fasting—no calorie
intake—induces a wide range of changes associated with
cellular protection, which would be difficult to achieve
even with a cocktail of potent drugs. In mammals, the
protective effect of fasting is mediated, in part, by an over
50% reduction in glucose and insulin-like growth factor 1
(IGF-I) levels. Because proto-oncogenes function as key
negative regulators of the protective changes induced by
fasting, cells expressing oncogenes, and therefore the
great majority of cancer cells, should not respond to
the protective signals generated by fasting, promoting the
differential protection (differential stress resistance) of
normal and cancer cells. Preliminary reports indicate that
fasting for up to 5 days followed by a normal diet, may
also protect patients against chemotherapy without
causing chronic weight loss. By contrast, the long-term
20 to 40% restriction in calorie intake (dietary restriction,
DR), whose effects on cancer progression have been
studied extensively for decades, requires weeks–months to
be effective, causes much more modest changes in glucose
and/or IGF-I levels, and promotes chronic weight loss in
both rodents and humans. In this study, we review the basic
as well as clinical studies on fasting, cellular protection and
chemotherapy resistance, and compare them to those on DR
and cancer treatment. Although additional pre-clinical and
clinical studies are necessary, fasting has the potential to be
translated into effective clinical interventions for the
protection of patients and the improvement of therapeutic
index.
Oncogene (2011) 30, 3305–3316; doi:10.1038/onc.2011.91;
published online 25 April 2011
Keywords: fasting; dietary restriction; chemotherapy;
differential stress resistance; starvation
Correspondence: Professor VD Longo, Andrus Gerontology Center,
Department of Biological Sciences and Norris Cancer Center,
University of Southern California, 3715 McClintock Avenue, Los
Angeles, CA 90089-0191, USA.
E-mail: [email protected]
Received 6 December 2010; revised and accepted 11 February 2011;
published online 25 April 2011
Introduction
For the past 50 years, chemotherapy has been a major
medical treatment for a wide range of malignancies
(Chabner and Roberts, 2005). Its main strategy has been
largely based on targeting malignant cells, by means of
genotoxicity caused in part by the production of reactive
oxygen species (Sangeetha et al., 1990; Look and
Musch, 1994; Faber et al., 1995; Conklin, 2004). In
fact, the US Food and Drug Administration has
approved 132 cancer chemotherapy drugs, of which 56
have been reported to cause oxidative stress (Chen et al.,
2007). Although these drugs were first believed to be
quite selective for tumor cells, we now know that normal
cells also experience severe chemotherapy-dependent
damage, leading to dose-limiting side effects including
myelosuppression, fatigue, vomiting, diarrhea and in
some cases even death. Chemoprotectants such as
amifostine, glutathione, mesna and dexrazoxane have
been investigated and shown to provide drug-dependent
protection to certain tissues, but these drugs have not
been shown to increase disease-free or overall survival,
in part because they may also protect cancer cells (Links
and Lewis, 1999). Despite the focused efforts on the
development of drugs designed to specifically target
certain cancer cells, side-effects will continue to accompany
cytotoxic drugs as well as a wide range of antibody-based
drugs until fundamentally novel strategies to selectively
eliminate malignant cells are discovered.
Fasting has been demonstrated to selectively protect
normal cells and mice, but not cancerous cells against
oxidants and common chemotherapeutic agents (Longo
et al., 1997; Raffaghello et al., 2008). In cancer patients,
preliminary data suggest that fasting is not only safe and
feasible, but may also be effective in reducing common
side-effects associated with chemotherapy (Safdie et al.,
2009). In this review, we discuss how short-term fasting
can selectively protect normal cells and organisms from
chemotherapy toxicity without interfering with its therapeutic effect (Figure 1), and also compare its effects with
those of the much better studied dietary restriction (DR)
(Table 1). We also discuss the evolutionarily conserved
role of the growth hormone (GH) and insulin-like growth
factor 1 (IGF-I) signaling pathways in stress resistance,
and how they act as major mediators of fasting-dependent
differential protection against chemotherapy and oxidative toxicity (Fontana et al., 2010). Finally, we discuss how
these studies may provide grounds for future investigations
Fasting vs dietary restriction in stress resistance
C Lee and VD Longo
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Figure 1 Fasting provides differential stress resistance (DSR) to
chemotherapy. The investment of the finite energy available in a
cell or an organism is efficiently balanced between growth/
reproduction and maintenance/repair. However, challenging conditions, such as fasting and its consequent reduction of IGF-I,
withdraws energy from growth/reproduction and reinvests it in
maintenance/repair, thereby increasing cellular protection. This
switch in cellular metabolic policy is mediated by negatively
regulating mitotic pathways such as those major effectors downstream of IGF-I (PI3K/Akt and Ras/ERK). By contrast, oncogenic
mutations, which often regulate cellular metabolism and growth,
render tumor cells less responsive to fasting due to their
independence from external cues. Therefore, cancer cells fail to or
only partially respond to fasting and continue to promote growth,
leaving them vulnerable to chemotherapy drugs.
on the role of various dietary interventions in enhancing
cancer treatment.
Starvation and stress resistance
Most organisms live in environments of fluctuating
food availability, where starvation is a commonly
encountered condition. Thus, nutrient depletion has
been an evolutionary driving force, selecting for organisms able to withstand starvation. Because during
periods of food scarcity organisms are exposed to a
wide variety of insults including UV radiation, heat,
cold and so on, adaptation to starvation requires the
organism to invest energy into multiple protective
systems to minimize the damage that would reduce
fitness (Madia et al., 2008, 2009). For example, yeast
switched from glucose and ethanol medium to water
becomes resistant to multiple types of stress and longlived (Longo et al., 1997; Fabrizio et al., 2005; Wei et al.,
2008). As discussed later, pro-growth pathways, including those activated by nutrients or growth factors, have
central roles in the prevention of the entry into this
protected mode.
DR is a term commonly used to describe a 20–40%
reduction in calorie intake, although it can also refer to
more or less severe restrictions or to reduced or lack
of daily intake of particular components of the diet,
such as amino acids, protein or fats. Although DR could
also describe the complete lack of food or calorie intake,
we will use the terms fasting or starvation to avoid
confusion with the more common use of DR. Both
DR and fasting have been shown to promote stress
resistance as well as longevity in model organisms
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ranging from unicellular yeast to mammals in part, by
downregulating conserved nutrient-signaling proteins,
or by activating stress resistance transcription factors
negatively regulated by these pro-aging pathways
(Figure 2) (Fontana et al., 2010). These pathways have
many regulatory effects including those on cellular
growth, metabolism and protection against oxidants
and other toxins.
The reactive oxygen species generated in response
to many chemotherapy drugs can cause a wide range
of genetic aberrations including DNA strand breaks and
base/nucleotide modifications (Burney et al., 1999).
Notably, a functioning immune system is required to
delay cancer growth or promote cancer regression in
response to several chemotherapy drugs, indicating that
oxidative damage may, in many cases, impair the cells
sufficiently to be detected and destroyed by the immune
system, thereby reducing chemotherapy efficacy (Locher
et al., 2010; Ma et al., 2010). For example, natural killer
T cells are required for the toxic effect of oxaliplatine
and irradiation on cancer cells, which also target rapidly
dividing normal immune cells (Locher et al., 2010).
One of the most common reactive oxygen speciesmediated DNA modification is the conversion of
guanine to 7, 8-dihydro-8-oxoguanine (also known as
8-oxoguanine; 8-oxoG) (David et al., 2007). This
damage can lead to mutations in nuclear and/or
mitochondrial DNA, but can also cause epigenetic
alterations, leading to cellular dysregulation and malignant transformation. Simple organisms such as yeast
and bacteria, in part because of their inability to escape
harsh environments, have evolved systems that can
increase resistance to many insults, in some cases by
several orders of magnitude. This protected mode
appears to be at least partially conserved in many
species. In the prokaryote E. coli, lack of glucose or
nitrogen (comparable to protein restriction in mammals)
increase resistance to high levels of H2O2 (15 mM)
(Jenkins et al., 1988). In many bacterial species,
starvation induces the stationary-phase-specific s factor
RpoS expression, which in turn controls a large set of
stress defense genes, in particular oxidative stress
(McDougald et al., 2002). In yeast, glucose reduction
(2–0.5%) increases protection against oxidative stress,
whereas complete starvation by switching the population of cells to water, promotes protection to oxidative
insults and heat shock and a major life span increase
(Longo et al., 1997; Wei et al., 2008). Although the
precise molecular mechanisms of increased protection
by fasting have yet to be described in detail, in yeast they
involve the reduced activity of the Ras/cAMP/PKA and
the Tor/S6K nutrient signaling pathways and the
activation of transcription factors downregulated by
these pathways (Thevelein and de Winde, 1999; Wei
et al., 2008). In fact, deletion of transcription factors
Msn2/Msn4 and Gis1 reverses the protection caused by
glucose restriction or starvation conditions, suggesting
an important role for genes involved in metabolism,
cellular protection and repair regulated by these factors
in starvation-dependent stress resistance (Longo et al.,
1997; Wei et al., 2008).
Fasting vs dietary restriction in stress resistance
C Lee and VD Longo
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Table 1
Fasting and dietary restriction (DR) induce similar, yet distinct physiological response in laboratory rodents
Fasting
Nutrient utilization
Glucose
DR
Effect
Reference
Effect
Reference
B50% k (Blood glucose)
(Wang et al., 2006;
Lee et al., 2010)
(Wang et al., 2006)
B43%; 25% k (Blood
glucose)
B70% k (Nitrogen
balance); B35% m (Plasma
ammonia), B63% m
(Uric acid)
B13% m (FFA), 96% k
(TG)
(Spindler et al., 1990;
Al-Regaiey et al., 2007)
(Felgines et al., 1999;
Filaire et al., 2004)
Protein
B50% m (Nitrogen excretion)
Lipids
100% m (FFA), 50% k (TG)
(Menahan and
Sobocinski, 1983)
B90% k
(Frystyk et al., 1999;
Wang et al., 2006)
(Frystyk et al., 1999;
Lee et al., 2010)
B13%; B50-fold m
B20-fold
(Shimokawa et al., 2003;
Zhao et al., 2010)
(Breese et al., 1991; Dunn
et al., 1997; Al-Regaiey
et al., 2007)
(Buschemeyer et al., 2010)
B20% k
(Buschemeyer et al., 2010)
GH/IGF-I and Insulin
GH
IGF-I
B40%; 70% k
IGFBP-1
B7-fold; 11-fold m
IGFBP-3
B50%; 40% k
B15%; 25%; 40% k
(Filaire et al., 2004)
Insulin
B90% k
(Frystyk et al., 1999;
Lee et al., 2010)
(Frystyk et al., 1999;
Lee et al., 2010)
(Frystyk et al., 1999)
B40% k
(Bonkowski et al., 2009)
Insulin-sensitivity
B3-fold m
(Heijboer et al., 2005)
B7/1.2-fold
(younger/older) m
(Escriva et al., 2007)
Corticosteroid
Body temperature
Reproduction
B2.5-fold m
B50% k
No fertility (a 48-h fast at
2nd day of estrous cycle)
(Shen et al., 2009)
(Shen et al., 2009)
(Wang et al., 2006)
B2-fold m
B12% k
Extended reproductive span
with reduced litter number
(B15%) and size (B50%)
(Sabatino et al., 1991)
(Koizumi et al., 1992)
(Longo and Finch 2003a, b;
Holehan and Merry, 1985)
Required time
2–3 days
(Raffaghello et al., 2008;
Lee et al., 2010; Mitchell
et al., 2009)
Weeks–months
(Longo and Finch 2003a, b;
Fontana et al., 2010)
Abbreviations: DR, dietary restriction; FFA, free fatty acid; GH, growth hormone; IGF-I, insulin-like growth factor 1; TG, triglycerides.
Reduction in glucose and increased utilization of protein and lipid occur following both fasting and DR, but more pronounced in the former
(Al-Regaiey et al., 2007; Felgines et al., 1999; Filaire et al., 2004; Lee et al., 2010; Menahan and Sobocinski 1983; Spindler et al., 1990; Wang et al.,
2006). The GH/IGF-I and insulin system are also significantly altered following fasting and DR. However, differences in GH response and also in
the degree of reduction in circulating IGF-I, IGFBP-3, insulin and in the increase of IGFBP-1 and insulin-sensitivity are evitable (Bartke and Turyn
2001; Bonkowski et al., 2009; Breese et al., 1991; Buschemeyer et al., 2010; Dunn et al., 1997; Escriva et al., 2007; Frystyk et al., 1999; Heijboer
et al., 2005; Lee et al., 2010; Shimokawa et al., 2003; Wang et al., 2006; Zhao et al., 2010). Corticosteroids tend to increase more following fasting
compared with DR (Sabatino et al., 1991; Shen et al., 2009); body temperature showed a steeper decrease following fasting compared with DR
(Koizumi et al., 1992; Longo and Finch 2003a; Shen et al., 2009). Fertility was decreased dramatically by fasting for 48-h and reached zero level
when done on the 4th day of the estrous cycle (McClure 1959). DR in rats, maintains body weight at 50% of control, reduced both litter number
and size (Holehan and Merry 1985). It should be noted that one of the major differences between fasting and DR is the time required to reach the
desired stress-resistant state, which makes fasting a much more applicable dietary intervention (Fontana et al., 2010; Lee et al., 2010; Longo and
Finch 2003a; Mitchell et al., 2009; Raffaghello et al., 2008).
Worms also increase their defenses against oxidative
insults during fasting. Every 2 days fasting increases
the resistance to oxidative stress, as well as lifespan of
worms up to 56% via modulation of the RHEB-1 and
TOR signaling pathway, both of which are linked to
the FOXO transcriptional factor homolog DAF-16
(Weinkove et al., 2006; Honjoh et al., 2009). Conversely,
excessive glucose shortens the lifespan of worms, in part,
by decreasing DAF-16 activity (Lee et al., 2009).
In flies, fasting-dependent protection against oxidative stress is mediated by d4E-BP, which acts downstream of the PI3K/Akt/dFOXO3 pathway (Tettweiler
et al., 2005). d4E-BP binds to eIF4E and represses
translation. During fasting, flies increase the expression
of d4E-BP, thereby suppressing eIF4B-dependent translation, a mechanism consistent with the necessity to
divert energy from growth to protection. Notably, flies
under moderate DR are not protected against oxidative
damage following anoxia/reoxygenation injury, whereas
a severe DR, that is near starvation, is able to reduce
damage (Vigne et al., 2009).
As discussed in more detail later, in mice, fasting
for 48–60 h increases protection from oxidative stress
and protects three different strains of mice to etoposide,
a drug known to promote oxidative stress, with
remarkable improvement in survival compared with
its normally fed counterparts (Raffaghello et al., 2008).
In addition, 72-h of fasting protects the outbred CD-1
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Fasting vs dietary restriction in stress resistance
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Figure 2 Fasting leads to a significant reduction in circulating
IGF-I levels. The partially conserved IGF-I signaling pathways
negatively regulate the FoxO family of transcription factors through
Akt. Ras and Tor also function downstream of IGF-I, although
their roles in the regulation of stress resistance and aging are poorly
understood. Mice deficient in type 5 adenylyl cyclase (AC) are
stress resistant (Yan et al., 2007). Notably, oncogenic mutations
that cause the hyperactivation of IGF-I, Akt, Ras, mTOR and
PKA are among the most common in human cancers (Hanahan
and Weinberg, 2000).
mice from lethal doses of doxorubicin, a drug also known
to cause death by oxidative stress-induced cardiotoxicity
(Lee et al., 2010). Studies also show that fasting protects
against ischemia injury in rat brain (Go et al., 1988;
Marie et al., 1990), mouse kidney and liver (Mitchell
et al., 2009; van Ginhoven et al., 2009), and human liver
(van Ginhoven et al., 2009). Also, fasting following
traumatic brain injury proved to be neuroprotective,
resulting in reduced oxidative damage and improved
cognitive function (Davis et al., 2008).
Conserved role of nutrient signaling pathways in stress
resistance
Nutrient signaling pathways controlled by insulin and/
or the GH/IGF-I axis are major regulators of both
lifespan and stress response as demonstrated by studies
in S. cerevisiae, C. elegans, D. melanogaster and mice
(Longo, 1999; Guarente and Kenyon, 2000; Kenyon,
2001; Longo and Finch, 2003b). In yeast, decreased Ras/
adenylate cyclase and/or Tor/S6K (SCH9) increases
lifespan by more than 200%, while providing increased
stress resistance against oxidants, genotoxins, and heatshock (Fabrizio et al., 2001, 2003; Longo and Finch,
2003b; Wei et al., 2009). Similarly, in C. elegans, age-1
(PI3K homolog) and daf-2 (insulin/IGF-I receptor
homolog) mutations extend the life-span in adult
organisms by 65–100% by decreasing AKT-1/AKT-2
signaling, and by activating the transcription factor
DAF-16 (FOXO family member), while promoting
resistance to oxidative and ER stress (Johnson, 1990;
Paradis et al., 1999; Hsu et al., 2003; Henis-Korenblit
et al., 2010). Also, DAF-16 is thought to be negatively
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regulated by PtdInsP3, which is regulated by the PI3K
homolog AGE-1; AGE-1 mutants have been shown to
be resistant to oxidative stress (Larsen, 1993; Morris
et al., 1996). These longevity mutations are largely
associated with the induction of stress resistance
transcription factors, superoxide dismutase (MnSOD),
and heat shock proteins (HSPs) in both yeast and worms
(Longo and Finch, 2003a; McColl et al., 2010). In
D. melanogaster, mutations in the insulin receptor
substrate (chico) extend lifespan, but its role in stress
resistance is not clear (Clancy et al., 2001; Giannakou
and Partridge, 2007). In mice, stress resistance and
lifespan are controlled by the GH/IGF-I axis and the
downstream orthologs of the genes that regulate stress
resistance and/or life span in yeast and worms (BrownBorg et al., 1996; Migliaccio et al., 1999; Coschigano
et al., 2000; Holzenberger et al., 2003; Yan et al., 2007;
Bonkowski et al., 2009; Selman et al., 2009). In agreement
with the findings in lower eukaryotes, the activities of
anti-oxidant enzymes superoxide dismutases and catalase
are decreased in murine hepatocytes exposed to GH
or IGF-I and in transgenic mice overexpressing GH
(Brown-Borg and Rakoczy, 2000; Brown-Borg et al.,
2002). In addition, cultured cells derived from long-lived
mice with deficiencies in the GH/IGF-I axis are resistant
to oxidative stress (H2O2, paraquat), UV, genotoxins
(methylmethanesulfonate; MMS), heat and cadmium
(Salmon et al., 2005; Murakami, 2006), even though
they are cultured in standard medium, suggesting that
some of the protective effects observed in long-lived
organisms may be also due to epigenetic changes
acquired during chronic exposure to reduced growth
factors.
In rats, IGF-I attenuates cellular stress response
and the expression of stress response proteins HSP72
and heme-oxygenase (Sharma et al., 2000). In primary
neurons IGF-I sensitizes cells to oxidative stress by a
Ras/Erk-dependent mechanisms (Li et al., 2008), and
experiments in rat primary glia and mouse fibroblasts
suggest that IGF-I sensitizes against oxidative damage
and chemotherapy drugs (Lee et al., 2010).
Together, the results from the range of studies and
organisms described above indicate that enhanced stress
resistance is a highly conserved phenotype of starved
and long-lived organisms, which is partially mediated by
downregulation of GH and IGF-I signaling.
DR and cancer treatment
Chronic moderate DR (20–40%) has been shown to
promote protective effects in a range of organisms.
Tumors rise from a combination of DNA damage and
mutations and changes in the environment surrounding
pre-cancerous cells (Hanahan and Weinberg, 2000). Not
surprisingly, aging, which promotes all the components listed above, is the major risk factor for cancer
(DePinho, 2000; Wedding et al., 2007; Campisi and Vijg,
2009; Jemal et al., 2010). It has been 100 years since the
first report on the direct relationship between the
amount of food intake and the growth or transplanted
Fasting vs dietary restriction in stress resistance
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tumors in mice showing that severe DR prevented
tumor transplantation (Moreschi, 1909). Since then, a
large body of work has established that DR reduces
the progression of tumors in various animal models
(Tannenbaum, 1945; Hursting et al., 1994). However,
recently it has been shown that specific oncogenes render
tumors unresponsive to DR, suggesting that the efficacy
of a reduction in food intake may be limited to a subset
of cancers (Kalaany and Sabatini, 2009).
DR has also been shown to reduce cancer incidence.
For instance, an ‘adult-initiated’ DR (‘undernutrition
without malnutrition’) beginning at 12 months of age
not only increases life-span, but also reduces spontaneous cancer incidence by more than 50% (Weindruch
and Walford, 1982; Weindruch et al., 1982). Furthermore, DR initiated at the time of weaning increased
lifespan and reduced tumor incidence in mice (Weindruch et al., 1986), confirming the DR studies in
rats. These studies have been extended to primates with
a 20-year longitudinal study in rhesus monkeys showing
that DR (30%) delays disease onset and mortality, in
part by causing a 50% reduction in cancer incidence
(Colman et al., 2009). Whether or not DR will reduce
cancer incidence in humans is still unclear. However,
DR can reduce clinical markers associated with
cancer, analogously to its effect in rodents (Longo and
Fontana, 2010).
The absence of DR in clinical cancer treatment after
so many decades of laboratory findings reflects the
limited potential of a chronic reduction in calorie intake
in cancer treatment. Among the problems associated
with the translation of DR into clinical applications
is that chronic DR delays but does not stop
the progression of the disease (Mukherjee et al., 2004;
Bonorden et al., 2009; Shelton et al., 2010), and that
this delay will occur for only a subset of malignancies
(Kalaany and Sabatini, 2009). Although weight loss
and cachexia in the early stages of cancer progression
are not as common as thought (Tisdale, 2002; Fearon
et al., 2003; Fox et al., 2009), the B15% loss of body
mass index caused by a moderate (20%) calorie
restriction (Racette et al., 2006) would prevent its
use in the great majority of cancer treatment scenarios.
In addition, the effect of long-term restriction in
delaying wound healing and impairing immune function (Fontana et al., 2010; Kristan, 2008; Reed et al.,
1996) may impose a significant risk to cancer patients
receiving chemotherapy, surgery or immunity-based
treatments (Kim and Demetri, 1996).
Intermittent fasting (IF), where fasting is applied
every other day, has been also shown to extend healthy
lifespan and reduce cancer incidence without causing
weight loss (Goodrick et al., 1983; Varady and
Hellerstein, 2007). However, the role of IF in combination with chemotherapy is not known. More importantly, because IF requires weeks–months to be
effective, it may be more useful for cancer prevention
than for cancer treatment. Nonetheless, it will be
important to compare the efficacy of IF with that of
prolonged fasting (short-term starvation), described in
the following section.
Fasting vs DR in cancer treatment
Even if the side effects and limitations of chronic DR
and IF were eliminated or reduced to a tolerable level,
initial data suggest that prolonged starvation in the 2–3
days range for mice and 4–5 days range for humans has
the potential to cause a much stronger protection of the
host as well as changes which may retard the growth of
cancer cells, without unwanted side effects. For example, the decrease in blood glucose caused by short-term
fasting in mice is 75% vs the 15% caused by longterm DR or IF (Heilbronn et al., 2005; Holloszy and
Fontana, 2007). Similarly, the 75% reduction in blood
IGF-I caused by a 2–5-day fast in mice and humans
(Clemmons and Underwood, 1991; Lee et al., 2010) is
not matched by DR, which causes a 25% IGF-I
reduction in mice (Barger et al., 2008) and does not
reduce IGF-I levels in humans unless protein intake is
also restricted (Fontana et al., 2008). Even together with
protein restriction, chronic DR only causes a B30%
reduction in IGF-I (Fontana et al., 2008).
The physiology of fasting
Although the effects of DR are often viewed as
responses that have evolved to adapt to a relatively
small (10–40%) reduction in food intake below the
normal laboratory food intake established for a
particular organism, it is more likely that they have
evolved to adapt to long periods of complete starvation.
Clearly, all organisms ranging from bacteria to humans
have undergone periods of starvation alternated with
periods of food availability. This is not only true for
bacteria or yeast that can spend weeks or months under
starvation conditions, but is also true for mammals.
In fact, humans, for example, have encountered many
and long periods of famine in their history and can
survive for over 1 month on water only. For example,
populations in Europe and Asia experienced frequent
and severe famines, which in some cases caused
hundreds of thousands of deaths (Scrimshaw, 1987).
The physiological response to fasting is quite different
from that to DR (Table 1). In mammals, there are three
metabolic stages during food deprivation (Wang et al.,
2006). First, the post-absorptive phase, which can last
for 10 or more hours following food ingestion and
involves the use of glycogen as the main stored energy
source. When the liver glycogen storage has been
depleted, it is followed by the second phase in which
amino acids serve as the substrate for gluconeogenesis.
Eventually glycerol and fatty acids released from
adipose tissues become the major source of energy.
The remaining glucose is mostly consumed by the brain,
and the newly fat-derived ketone bodies (that is
acetoacetate, b-hydroxybutyrate and acetone) become
the major carbon sources in a matter of days (Cahill,
1970; Cahill et al., 1970; Cahill, 2006). Following
prolonged fasting lasting a week or longer, fat-derived
b-hydroxybutyrate becomes the most abundant ketone
body, accounting for roughly two thirds of brain fuel,
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and glucose production reaches a very modest level
(Cahill, 2006). During the last phase of prolonged food
deprivation, fat reserves are eventually exhausted and
rapid muscle degradation occurs to fuel gluconeogenesis. In rats, this occurs after 4–5 days but does not
result in a major increase in glucose levels (Wang et al.,
2006). Weight loss caused by fasting is initially rapid but
subsequently tapers off as shown in a human study, in
which an average of 0.9 kg/day was lost during the first
week of fasting, followed by a 0.3 kg/day weight loss by
the third week of fasting. This study also reported an
approximately 20% body weight loss caused by 30–35
days of fasting (Kerndt et al., 1982). It is estimated that
a 70 kg person can obtain basal caloric requirements
from fat reserves during up to 2–3 months of fasting
(Cahill and Owen, 1968; Cahill et al., 1968; Saudek and
Felig, 1976). Thus, prolonged fasting is feasible and
generally well tolerated in humans, but may be
accompanied by relatively minor side-effects, such as
headaches, light-headedness, nausea, weakness, edema,
anemia and amenorrhea (Bloom, 1959; Drenick et al.,
1964; Thomson et al., 1966). In some rare cases, extreme
fasting for periods of several weeks or longer in obese
subjects has been reported to cause fatal complications
such as renal failure, heart failure and lactic acidosis
(Cubberley et al., 1965; Spencer, 1968; Garnett et al.,
1969; Runcie and Thomson, 1970). Prolonged fasting
can also cause severe problems and possibly death, as
it occurred in some prisoners of concentration camps
after they were liberated, if the re-feeding period is not
gradual. Further, fasting should be limited to subjects
that do not have deficiencies in metabolic pathways. For
instance, patients with deficiencies in gluconeogenesis
(for example, mutation in fructose-1,6-bisphosphatase),
are known to experience hypoglycemia, acidosis, as well
as failure to produce ketone bodies during fasting,
rendering them highly susceptible to this severe restriction (van den Berghe, 1996).
Adaptation to starvation also occurs at the cellular
level. A key survival mechanism a cell employs is selfeating, or autophagy, which is also activated in response
to cellular stress (for example, chemotherapy) in various
eukaryotes. For example, starving mammalian cells of
essential nutrients (for example, glucose) and growth
factors (for example, serum) induces autophagy, and
starving mice for 12–24 h triggers autophagy in various
tissues (Kroemer et al., 2010; Marino and Kroemer,
2010). Interestingly, autophagy is regulated by several
genes, such as AMPK, mTOR, and sirtuins, which are
also known to regulate aging and stress resistance
(Morselli et al., 2009; Kroemer et al., 2010; Marino and
Kroemer, 2010). Many oncogenic signals (for example,
PI3K and Akt) inhibit autophagy, whereas tumor
suppressors (for example, PTEN and TSC2) trigger
autophagy. Surprisingly many cancer cells display
elevated autophagy, an increase which was recently
suggested to be mediated by ammonia, a by-product
of glutaminolysis that preferentially occurs in the
mitochondria of malignant cells undergoing the Warburg effect (Vander Heiden et al., 2009; Marino and
Kroemer, 2010). Under the Warburg effect, mitochonOncogene
dria takes on a different role to supply biosynthetic
precursors that are required for the rapid proliferation
rate set by oncogenic mutations (Vander Heiden et al.,
2009). Therefore, autophagy triggered by fasting may be
beneficial to normal cells but detrimental to malignant
cells. In fact, considering that autophagy increases the
resistance of cancer cells to chemotherapy (Marino and
Kroemer, 2010), it will be important to determine
whether fasting promotes cancer sensitization alone or
in combination with fasting.
Fasting, glucose and growth factors
Glucose levels undergo remarkable changes during food
restriction, and can reach 50–60 mg/dl in a healthy person
after only 72 h of fasting, but return to normal levels
within 30 min of administering 100 g of glucose orally
(Unger et al., 1963). Fasting also reduces a plethora of
extrinsic and intrinsic growth factors, particularly the
insulin-like growth factor 1 (IGF-I), which acts as the
major growth effector of growth hormone. If both the
reduction and the rate of decrease are considered, the
effect of fasting on the GH/IGF-I axis is unlikely to be
surpassed by any other intervention including Food and
Drug Administration approved drugs such as pegvisomant, a GH receptor antagonist. As described earlier, the
GH/IGF-I axis is also a major regulator of metabolism
and activates evolutionarily conserved pathways that
promote stress sensitivity and aging (Fontana et al.,
2010). In humans, IGF-I levels decrease dramatically in
response to a short-term starvation (36–120 h) despite
increased GH secretion, which is highly lipolytic (Merimee
et al., 1982; Isley et al., 1983; Thissen et al., 1999; Maccario
et al., 2001; Norrelund, 2005; Moller and Jorgensen, 2009).
This effect may be the result of a major increase in the
IGF-I inhibitory protein IGFPB-1 (Zapf, 1995), which
decreases further IGF-I bioavailability and prevents the
feedback inhibition of GH secretion by IGF-I (Muller
et al., 1999). Notably, the 65% reduction in IGF-I levels
induced by fasting for 5 days persisted 24 h after refeeding, and only reached 70% of normal levels even 5
days after normal feeding (Isley et al., 1983). In non-obese
subjects GH levels eventually level off and drop between
3–10 days of fasting (Cahill et al., 1966; Merimee and
Fineberg, 1974; Palmblad et al., 1977). In mice, a shortterm starvation (24–72 h) decreases IGF-I production by
70% and causes an 11-fold increase in its inhibitory
partner IGF-binding protein 1 (IGFBP-1) (Tannenbaum
et al., 1979; Frystyk et al., 1999; Lee et al., 2010). In
response to fasting, mice also show reduced protein
synthesis, reduced AKT activity via TRB3, a mammalian
homolog of Drosophila tribbles (Du et al., 2003), reduced
mTOR/S6K, increased 4E-BP1 activity (Sans et al., 2004)
and increases FOXO-1, -3, -4 (Imae et al., 2003).
Differential stress resistance (DSR) by fasting
Whereas the inactivation of partially conserved proaging pathways increases resistance to oxidative and
Fasting vs dietary restriction in stress resistance
C Lee and VD Longo
3311
Table 2 Fasting protects against chemotherapy toxicity, in part by
reducing IGF-I levels
Drug
Mode of action
Effect of fasting and/or
reduced IGF-I
Doxorubicin
DNA intercalation
Cyclophosphamide
DNA alkylation
5-Fluorouracil
Anti-metabolite
Etoposide
Topoisomerase II
inhibition
Protection by fasting
and reduced IGF-I
(mice)
Protection by reduced
IGF-I (mice)
Protection by reduced
IGF-I (mice)
Protection by fasting,
but sensitized by reduced IGF-I (mice)
Docetaxel
Microtubule
stabilization
Carboplatin
DNA alkylation
Paclitaxel
Microtubule
stabilization
Gemcitabine
Anti-metabolite
Reduced side-effects by
fasting (human)
Abbreviation: IGF-I, insulin-like growth factor 1.
As tested in mice, fasting and the reduction of circulating IGF-I
protect against doxorubicin, cyclophosphamide and 5-fluorouracil (5FU) (Lee et al., 2010; Raffaghello et al., 2008). Notably, fasting
protected against etoposide, but the reduction of IGF-I alone showed
opposite results, suggesting a more complex role of fasting beyond
IGF-I in the protection against certain drug categories. In a recent case
series, patients that voluntarily fasted during chemotherapy reported
significantly reduced side-effects (Safdie et al., 2009). In several cases,
chemotherapy was limited by toxic side-effects, but upon fasting,
patients were able to resume normal treatment schedule. The
chemotherapeutics administered to these patients include a combination of docetaxel, carboplatin, paclitaxel and gemcitabine.
other stresses in lower eukaryotes, the constitutive
activation of analogous pathways is known to promote
cancer. In fact, both Ras and Akt, whose orthologs are
central in stress sensitization in yeast and worms,
function in signal transduction pathways that are among
the most frequently found in a constitutively activated
form in human cancers (Medema et al., 1993; Kinzler
and Vogelstein, 1996; Hanahan and Weinberg, 2000).
This connection between cancer and stress resistance
provides the theoretical foundation for the differential
killing of cancer cells not by the identification of drugs
that specifically kill malignant cells, but by the activation of stress resistance in normal cells (differential stress
resistance section, DSR) and the treatment with
chemotherapy (Table 2). DSR is based on the hypothesis that in response to fasting, normal cells, independently of the cell type, will enter an alternate state
characterized by reduced or lack of cell division, a
switch to the utilization of the metabolites generated
from the breakdown of fats, proteins and organelles
(autophagy) described earlier, and resistance to multiple
stresses (Longo et al., 1997; Longo and Finch, 2003a;
Raffaghello et al., 2008). The inability of cancer cells to
respond to anti-growth signals or to grow in the absence
of growth factors is a well-established hallmark of
cancer (Hanahan and Weinberg, 2000; Luo et al., 2009).
More specifically, self sufficiency in growth signals is
enabled by gain-of-function mutations in oncogenes (for
example, Ras, Akt, mTOR and so on) that grant
constitutive activation of proliferation pathways regardless of conditions. On the contrary, insensitivity to
growth inhibitory signals is due to loss-of-function
mutations in tumor-suppressor genes (for example, Rb,
p53, PTEN and so on), allowing cancer cells to disregard
anti-proliferation signals (Hanahan and Weinberg,
2000; Vogelstein and Kinzler, 2004) and therefore also
disregard the fasting-induced signals.
Thus an effect of fasting on the growth of normal but
not of cancer cells alone provides a method to
preferentially target malignant cells with chemotherapy
(Raffaghello et al., 2008). However, this effect could be
greatly enhanced by increasing stress resistance and
activating a number of cell detoxification systems in
both dividing and non-dividing cells. In fact, protection
against stress in yeast mutants lacking RAS2 and SCH9
that are also starved increases 1000-fold or more
compared with the approximately 10-fold increase in
protection provided by fasting alone (Raffaghello et al.,
2008). Because, the inactivation of pro-growth pathways, particularly multiple pathways activated by
glucose and amino acids, can increase stress resistance
in both yeast and mammalian cells, the expression of
constitutively or partially active oncoproteins present in
the great majority of tumors is expected to prevent the
protective effect of fasting regardless of the type of
cancer or oncogene. Furthermore, cancer cells have been
known for decades to rely more on glycolysis than on
oxidative phosphorylation (Warburg effect), raising the
possibility that the fasting-based enrichment of ketone
bodies may increase their sensitivity.
The Warburg effect does not only apply to cancer
cells, but also to rapidly dividing normal cells such as
lymphocytes, thymocytes and enterocytes (Newsholme
et al., 1985). Although oxidative phosphorylation is far
superior to glycolysis in terms of ATP production,
glycolysis provides biosynthetic precursors that are
essential to rapidly dividing cells. For instance, during
glycolysis, glucose-6-phosphate can be directed to the
pentose shunt pathway, providing reducing power and
substrate for nucleotide synthesis, and glycerol can be
processed into phospholipids that contribute to the cell
wall (Alberts et al., 2002). Increased glucose metabolism also allows cancer cells to restrict cytochrome
c-mediated apoptosis, further favoring tumor cell survival
(Vaughn and Deshmukh, 2008). The Warburg effect
may encourage apoptosis evasion by also decreased
respiration. It has recently been shown, using yeast as a
model organism, that respiration during seeding and
development of a cell population triggers apoptosis,
suggesting that decreased respiration may contribute to
tumorigenesis (Ruckenstuhl et al., 2009).
A major concern regarding the effect of fasting in
improving cancer treatment is the possibility that it may
also protect malignant cells. However, a number of
studies in simple model organisms, mammalian cells and
mice indicate that oncogenes can render malignant cells
unresponsive to fasting, therefore allowing fasting to not
interfere with therapeutic efficacy in the great majority
of cancer types. As mentioned above, in yeast, the
Oncogene
Fasting vs dietary restriction in stress resistance
C Lee and VD Longo
3312
combination of starvation (switch from glucose medium
to water at day 1) with the deletion of SCH9/S6K or
both SCH9/S6K and RAS2 increased resistance to
treatment with hydrogen peroxide or menadione a
1000-fold, compared with that of cells expressing the
constitutively active oncogene homolog RAS2val19 or
cells lacking SCH9/S6K (sch9/s6kD) but expressing
RAS2val19 (sch9/s6kDRAS2val19) (Raffaghello et al.,
2008). In fact, the expression of the oncogene-like
RAS2val19 can not only reverse the protective effects of
starvation and mutations, but also reduce the resistance
of cells compared with wild-type cells, raising the
possibility that fasting may have a sensitizing effect on
cancer cells. In mice, using a metastatic mouse model of
neuroblastoma, induced by intravenous injection of
NXS2 cells, fasting effectively protected the mouse
without interfering with chemotherapy efficacy, suggesting that it did not protect these cancer cells (Raffaghello
et al., 2008). In this study, tumor-bearing mice were
fasted for 48 h followed by a single administration of
high-dose chemotherapy, which successfully improved
survival limited by both drug toxicity and metastases.
Analogously to the activation of yeast Sch9 and Ras
by amino acids and glucose, the mammalian IGF-I
receptor activates PI3K/Akt/PKB and Ras, and regulates glucose metabolism and cellular proliferation
(Kandel and Hay, 1999). PI3K, which is frequently
found mutated in human cancers (Yuan and Cantley,
2008), regulates growth and glucose metabolism in part
via its downstream effector AKT (Vander Heiden et al.,
2009). Notably, in normal cells, fasting can reduce PI3K
signaling (Xie et al., 2007), leading to the protection of
the cell. mTOR is another well-established master
regulator, which integrates three major components—
growth factors, nutrients and cellular energy level
(ATP)—to regulate cellular growth and metabolism.
Therefore, whereas reduced glucose can protect normal
cells (Raffaghello et al., 2008) it may be detrimental to
the survival of malignant cells.
Reduced IGF-I levels mediate in part fasting-dependent
DSR
As mentioned above, fasting causes a 70% decrease in
circulating IGF-I levels, and restoring its levels during
fasting reverses the protection against doxorubicin in
mice (Lee et al., 2010). The effect of reduced IGF-I
on protection against chemotherapy toxicity was also
studied by using transgenic mice with a conditional
liver-specific igf-1 gene deletion (LID), which results in a
70–80% reduction in circulating IGF-I levels, analogously to that caused by fasting. LID mice showed
enhanced protection to commonly used chemotherapy
drugs cyclophosphamide, doxorubicin and 5-FU,
although they were not protected against the topoisomerase inhibitor etoposide. Reducing IGF-I signaling
may provide dual benefits by protecting the organism
and reducing tumor progression. In fact a recent study
demonstrated the central role of IGF-IR in tumor
progression and regression (Jones et al., 2010) and at
Oncogene
least 12 different IGF-IR targeting compounds, including small antagonistic molecules and antibodies, have
entered clinical trials (Gualberto and Pollak, 2009).
Fasting in clinical applications
Historically, fasting was performed for both medical
and religious purposes (Kerndt et al., 1982; Michalsen
et al., 2005; Johnstone, 2007). Much has also been
learned about the effects of fasting from data on the
victims of famine and war (Scrimshaw, 1987; Kalm and
Semba, 2005), and also data from subjects fasting
voluntarily for 40 days or longer (Kerndt et al., 1982).
Beyond its traditional practice, fasting has been
demonstrated to have clinical benefits. Notably, clinical
studies have shown that water-only fasting for 10–14
days significantly improved hypertension by reducing
systolic blood pressure points more than two-fold
compared with that of a combined vegan, low-fat,
low-salt diet and exercise (Goldhamer et al., 2001;
Goldhamer, 2002). Moreover, the safety of fasting in
patients with chronic disease has been studied in a large
cohort study with over 2000 participants (Michalsen
et al., 2005). In this study, the authors determined that
fasting (350kcal/day) was safe and considered by many
of the participating subjects to be beneficial to their
chronic disease (Michalsen et al., 2005).
In a recent report, 10 patients with a variety of
malignancies voluntarily fasted for up to 180 h in combination with chemotherapy, and reported a reduction
in common chemotherapy-associated side-effects such as
vomiting, diarrhea, fatigue and weakness (Safdie et al.,
2009) (Table 2). Notably, in the cases where cancer
progression could be followed, there was no evidence
that fasting protected tumors or interfered with chemotherapy efficacy. A controlled randomized clinical trial
testing the effect of fasting on cancer treatment underway
at the University of Southern California Norris Cancer
Center is expected to provide more conclusive clinical data
on the effect of fasting on chemotherapy toxicity and
efficacy.
Conclusion and discussions
According to the American Cancer Society, cancer
patients receiving chemotherapy should increase calorie
and protein intake (Doyle et al., 2006). Contrary to this
dogma, many studies suggest that a 20–40% reduction
in calorie intake protects the host against toxins and
retards the growth of tumors (Moreschi, 1909; Tannenbaum, 1945; Hursting et al., 1994, 2001, 2003; Fontana
et al., 2001, 2006; Harper et al., 2006; Bonorden et al.,
2009; Colman et al., 2009; Longo and Fontana, 2010).
However, this has never been translated into clinical
applications because the effects are limited, as reduced
calorie intake unavoidably causes weight loss, and
because it was not clear that DR could not also protect
cancer cells from chemotherapy. By contrast, based on
the DSR studies and on clinical data, fasting for a
Fasting vs dietary restriction in stress resistance
C Lee and VD Longo
3313
limited period can have a potent protective effect on the
host and possibly detrimental effects on a variety of
tumors. However, fasting still poses a challenge to
patients, which calls for an urgent need of substitution
diets or targeted drugs that can mimic fasting. Experimental evidence from aging research suggests that diets
with specific deficiencies, especially essential amino
acids, may provide increased stress resistance and
lifespan (Richie et al., 1994; Takenaka et al., 2000;
Harper et al., 2006). Although, it is unlikely that the vast
effects caused by fasting can be obtained with specific
deficiencies, it will be important to determine whether
limitations of specific nutrients will be sufficient to
promote some of the effects of fasting. Furthermore,
because cancer depends on certain nutrients, there is a
large potential for the combination of dietary interven-
tions and chemotherapy or other non-toxic cancer
therapies for the enhancement of cancer treatment. In
addition, as discussed in this study, exploring drug
targets based on the IGF-I and related systems may also
identify fasting-mimetics that provide DSR. Interventions that could provide a differential protection of host
and cancer cells in the 1000-fold differential protection
range observed in yeast cell lacking or expressing an
oncogene analog, could have paradigm shifting effects in
cancer treatment.
Conflict of interest
Valter Longo founded L-Nutra, which develops diets
for cancer patients.
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